1. Field of the Invention
The present invention relates to an optical disc, and more particularly, to a high density read only optical disc that allows reading of marks (pits) having a size not greater than a reading resolution limit of a laser beam by using a super-resolution near-field structure placed inside the disc.
2. Description of the Related Art
According to the characteristics of discs, optical discs are classified into read only memory (ROM) optical discs that allow only reading of information recorded thereon, write once read many (WORM) or recordable optical discs that allow information to be recorded thereon only once, and erasable or rewritable optical discs that allow information to be erased and re-recorded thereon.
A compact disc recordable (CD-R) is an example of the WORM optical disc. The CD-R includes a recording layer comprising an organic dye such as cyanine, phthalocyanine, /or the like. A recording laser with a wavelength of 780 nm is irradiated onto the recording layer to decompose the dye layer, and a recorded signal may be read with a power of 1 mW or lower. The CD-R has a recording capacity of about 650 MB and is widely used for recording and reading various types of data such as text data, music, images, and/or the like.
However, optical recording media such as the CD-R, or a compact disc rewritable (CD-RW) using a recording laser beam with a wavelength of 780 nm, have insufficient capacity to store dynamic images, and thus cannot be used in a complex multimedia environment.
To resolve the above problems, digital versatile discs (DVDs) having a capacity of 2.6–4.7 GB by using laser beams with a short wavelength of 630–680 nm have been developed. The DVDs are also classified into readable only DVD (DVD), write once read many DVD (DVD-R), and erasable DVD (DVD-RAM, DVD+RW, DVD-RW). Regarding the DVD-R, a recording laser beam is irradiated onto a recording layer to transform and decompose the recording layer, thus recording data. Regarding the DVD-RAM and DVD-RW, a change of the optical characteristic is caused by a change of phase, thus recording data. Since the DVD-R using an organic dye is more advantageous than other media in terms of compatibility with a DVD-ROM, price, and capacity, research is concentrated on developing the DVD-R.
Regarding many media developed recently, the biggest issue is capacity and various methods to increase the capacity are being researched. Information recorded on the read only optical disc is previously formed on a substrate as pits. Upon reading the information, a laser beam is irradiated onto the optical disc by a reading apparatus, and strength and weakness of reflected beam by the pits is measured using a photodetector. For example, when there is a pit, the reflected beam is weak, and when there is no pit, the reflected beam is strong. The amount of information capable of being recorded on the read only optical disc predominantly depends on how many accurately readable and small pits are formed in a fixed area and characteristics of a laser beam capable of accurately reading the pits. Although light emitted from the laser diode is focused through an objective lens, the light is not collected to one infinitely small point due to the effect of diffraction and forms a beam having a finite width, which is called the diffraction limit. In the case of general optical discs, when the wavelength of a light source is λ, and the numerical aperture of the objective lens is NA, λ/4NA represents a reading resolution limit. Thus, as the wavelength of light source shortens, or the numerical aperture of the objective lens increases, the recording capacity increases. However, current laser technologies cannot provide lasers with a short wavelength, and the cost of manufacturing an objective lens with an increased numerical aperture is high. Further, since as the numerical aperture of an objective lens increases, the working distance between the pickup and the disc considerably shortens, the surface of the disc may be damaged due to collision of the pickup with the disc. As a result, data may be lost.
In the case of a red laser, which is generally used in the art, since a wavelength is 635 nm and NA is 0.6, the reading resolution limit is 265 nm.
FIG. 1 is a graph illustrating the relationship between a length of a pit and carrier to a noise ratio (CNR) in a conventional read only disc having a reflective silver layer formed on a substrate. As is apparent from FIG. 1, when the pit length is not less than 300 nm, the CNR is not less than 40 dB, and information recorded as pits is smoothly read. However, when the pit length is less than 300 nm, the CNR sharply decreases, and when the pit length is not greater than 250 nm, the CNR is close to 0.
To overcome the reading resolution limit, an optical disc having the super-resolution near-field structure (hereinafter referred to “super-resolution structure”) has recently been researched. Regarding the super-resolution structure, a special mask layer is formed in an optical disc, and when reading information, the surface plasmon generated in the mask layer is used. For example, in the case of an optical disc having a mask layer of silver oxide, the silver oxide is decomposed into silver and oxygen, and the decomposed silver generates the surface plasmon. Near field reading is possible by the surface plasmon, and the reading resolution limit problem is overcome, so that a very small recording mark may be read.
FIG. 2 schematically illustrates a structure and a recording principle of the conventional recordable optical disc 100 having the mask layer 11 made of silver oxide. As shown in FIG. 2, the recordable optical disc 100 includes a dielectric layer 14 comprising, for example, ZnS—SiO2, the mask layer 11 comprising, for example, AgOx, wherein x is a positive number, a dielectric layer 14 comprising, for example, ZnS—SiO2, a recording layer 12 comprising, for example, GeSbTe, a dielectric layer 14 comprising, for example, ZnS—SiO2, and a reflective layer 13, which are sequentially stacked upon one another on a substrate 10.
A laser beam with a power of about 10–15 mW is irradiated onto a recording layer 12 to record information. In this case, silver oxide is decomposed into fine metal particles and oxygen. Upon reading, plasmon is formed on a surface of the fine silver particle. However, since information is recorded on the substrate 10 in the form of a pit without the recording process in case of a read only optical disc, the decomposition of the silver oxide into silver particles and oxygen does not occur. Thus, it is difficult to select a structure and a material of a disc that facilitates recording on the read only optical disc using only a laser beam with low power as effectively as a disc having the super-resolution structure.